1. Early Damage Mechanisms in Nuclear Grade Graphite under Irradiation Jacob Eapen, Ram Krishna, T. D. Burchell † and K. L. Murty Department of Nuclear Engineering North Carolina State University, Raleigh, NC 27695 † Carbon Materials Technology Group Oak Ridge National Laboratory, Oak Ridge, TN 37831 9/16/20131

2. 2 Objective and Summary Disordering mechanisms in graphite have conflicting view points. We use Raman spectroscopy, XPS and TEM to investigate the early damage mechanisms in NBG-18 under neutron/ion irradiation. Our results show evidence for topological defects under irradiation, even at high doses. Amorphization by direct collapse of vacancies is deemed unlikely, – instead it is likely mediated through multiplication of dislocations.

3. 39/16/2013 Disordering Mechanisms: Traditional View Large number of interstitial and vacancies are generated. Point defects agglomerate – collapse into dislocation loops. Additional planes are formed between graphitic layers leading to expansion perpendicular to basal plane (c-axis). In nuclear graphite collapse of micro-cracks initially masks the expansion along c-axis. Expansion: c-axis Contraction: a-axis

4. 49/16/2013 Disordering Mechanisms: Experimental Evidence Bending and warping of basal planes are observed. Strong evidence for formation of new planes/sheets from interstitials is lacking. Current work:  Neutron irradiated NBG-18  0.002 dpa  325 K Heggie, S.–Martinez, Davidson, Haffenden J. Nuclear Materials, 413 p. 150 (2011)

5. June 18, 2013 ANS Meeting - Atlanta 5 Disordering Mechanisms: Experimental Evidence Koike & Pedraza, J. Mater. Res. 1994;9(7):1899-1907. Tanabe et al, App. Phys. Lett. 61 p. 1638 (1992) e - irradiation, no interstitial loops, large expansion along c-axis. Bending and fragmentation of basal planes into nanocrystallites. Partial-to-full amorphozation, at low temperatures and high dpa.

6. 69/16/2013 Amorphization Mechanisms Disordered Region Model: The defect (D) peak in Raman spectra is correlated to vacancy or vacancy clusters. Also mentions ‘in-plane’ defects. On saturation, vacancies transform into disordered and amorphous regions (beyond a critical irradiation dose). Niwase, Physical Review B. 52, p. 15785 (1995)

7. 79/16/2013 Amorphization: Two Mechanisms Dislocation Accumulation Model: Frenkel pairs generated by radiation give rise to divacancies. They morph into stable dislocation dipoles that multiply with increasing irradiation dose. Niwase, Phil. Mag. Lett, 82, p. 401 (2002) Niwase, Int. J. Spectroscopy, ID: 197609 (2012)

8. 89/16/2013 Disordering Mechanisms: Recent Progress Karthik, Kane, Butt, Windes and Ubic J. Nuclear Materials, 412 p. 321 (2011) Vacancy loops dissociate into prismatic dislocations. Incomplete planes, formed by climb mechanism, lead to expansion along c-axis

9. 99/16/2013 Disordering Mechanisms: Recent Progress Heggie, S.–Martinez, Davidson, Haffenden J. Nuclear Materials, 413 p. 150 (2011) Basal edge dislocation (a) sweeping right to left, (b) climbs a plane, and (c) extends the ruck and tuck defect. (d) DFT simulation result. Ruck and Tuck Mechanism

10. 109/16/2013 Current Work Neutron Irradiation  Low dpa: PULSTAR Reactor, NC State University.  Fast neutron flux: 2×10 12 n/(cm 2 -s)  dpa = 0.002 – 0.01 dpa  Temperature: 325 K  High dpa: Oak Ridge Research Reactor (ORR).  dpa: 6.6/10.1 dpa Ion Irradiation  University of Wisconsin through NSUF  dpa:1–25  Temperature: 300 K – 900 K

11. 119/16/2013 High Temperature Materials Testing Capsule at NC State

12. 129/16/2013 Results: Raman Spectra Eapen, Krishna, Burchell and Murty, Materials Research Letters, In Press (2013)

13. June 18, 2013 ANS Meeting - Atlanta 13 Raman Spectra of Ion Bombarded Mono-layer Graphene G peak: arises from atomic vibrations. G’ peak: overtone of G peak. D peak: disorder peak arising from breathing modes of closed rings. D peak can emerge only from sp 2 bonds. D’ peak: minor defect peak. L D : measure of the amount of disorder; the distance between defected regions. Canc ̧ ado et al. Nano Lett. 11(8), p. 3190 (2011)

14. 149/16/2013 sp 2 and sp 3 Bonds sp 2 sp 3

15. 159/16/2013 Interpretation of Peaks in Raman Spectra Solid-state Picture and the Molecular Picture  G Peak:  Stems from a single resonance process.  Corresponds to E 2g phonon scattering with zero momentum at Γ point.  Proportional to the sp 2 carbon sites.  Represents bond stretching of sp 2 sites.  It occurs at the same Raman shift for defected and pristine crystalline structures. Ferrari and Robertson, Phys. Rev. B. 61, p. 14 095 (2000)

16. 169/16/2013 Interpretation of Peaks in Raman Spectra Solid-state Picture and the Molecular Picture  D Peak:  Stems from a higher order, double resonance process.  Excitation of momentum q≠0 process: allowed for defected samples but not for crystalline structure.  Stems from the A 1g breathing mode.  No rings – No D peak. Ferrari and Robertson, Phys. Rev. B. 61, p. 14 095 (2000), Phil. Trans. R. Soc. Lond. A 362, p. 277 (2004).

17. 179/16/2013 Topological Defects Maintain sp 2 connectivity  Stone-Wales Defect:  Created by rotation of bonds.  No bonds are broken.  More Complex Defect Types

18. 189/16/2013 Topological Defects More Complex Defect Types  Created by rotation of bonds, without broken bonds.  maintain sp 2 bond structure, connectivity.  Interacts with point defects.

19. 199/16/2013 Topological Defects

20. 209/16/2013 Changes to Bond Structure with Irradiation Ferrari and Robertson, Phil. Trans. R. Soc. Lond. A 362, p. 277 (2004).

21. 219/16/2013 Changes to Bond Structure with Irradiation

22. 229/16/2013 Changes to Bond Structure with Irradiation Telling, Ewels, El-Barbary and Heggie, Nature, 2, p. 333 (2003) sp 2 sp 3

23. 239/16/2013 Raman Spectra at 25 dpa with Ion Irradiation

24. June 18, 2013 ANS Meeting - Atlanta 24 Transmission Electron Microscopy at High Resolution (HRTEM) Virgin NBG-18 Irradiated NBG-18 (0.002 dpa)

25. June 18, 2013 ANS Meeting - Atlanta 25 Evidence of Dislocation Loops & Partial Dislocations in NBG-18 (b) (a) (d) (c) Basal Dislocations splitting into partials

26. 269/16/2013 Summary We use Raman spectroscopy, XPS and TEM to investigate the early damage mechanisms in NBG-18 under neutron/ion irradiation. Our results show evidence for topological defects under irradiation, even at high doses. Amorphization by direct collapse of vacancies is deemed unlikely, – instead it is likely mediated through multiplication of dislocations.

27. 9/16/201327 Thank you!